Cash Deposits and Other Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]
Insurance collateral deposits	$ 3,517	$ 3,767
Excess loss fund	2,093	2,284
Accumulated interest income	642	814
State collateral deposits	57	57
Materials and supplies	273	273
Other	36	37
Total	$ 6,618	$ 7,232